The present invention relates to a lithium secondary battery, and more particularly to its electrode.
The lithium battery, in particular, the rechargeable lithium secondary battery is intensively researched and developed recently as a new secondary battery having high voltage and high energy density. In earlier studies, the lithium secondary battery using lithium metal in the negative electrode has been highly expected as the battery of high energy density. However, when lithium metal is used in the negative electrode, dendric lithium formed at the time of charging is grown in the course of charging and discharging of battery, which leads to internal short circuit of battery or abnormal temperature rise of battery. Such safety problems have not been solved yet.
To solve these problems, instead of using lithium metal alone, it has been attempted to use an alloy of lithium metal with low melting metal such as aluminum, lead, indium, bismuth or cadmium, as the negative electrode. In these cases, however, as charging and discharging are repeated, pulverized alloy penetrates through the separator to cause internal short circuit, and it is far from practical and the problems are not solved.
Recently, to solve the problems, using carbon in the negative electrode, the lithium secondary battery using transition metal compound containing lithium in the positive electrode is coming into the mainstream. In this battery system, since charging and discharging are done by occlusion and release of lithium ions into carbon in the negative electrode, dendrite is not formed by charging. Hence, the battery has an excellent cycle characteristic and is excellent in safety.
In the existing lithium secondary battery, as mentioned above, carbon is used as active substance in the negative electrode, and charging and discharging are effected by occlusion and release of lithium ions into carbon. When powder is used as active substance, required conditions of the negative electrode of the battery include, aside from the lithium ion occluding and releasing capacity of the carbon itself, the filling performance of filling the limited volume of the battery with how much carbon. In the lithium secondary battery, usually, a mixed paste of carbon and adhesive is applied on both sides or one side of thin metal films used as current collectors, and the obtained plates are dried and rolled to form electrodes. In the plates of high capacity type having such high filling performance, it is a technical problem to accelerate ion conduction in the limited gap existing in the active substance grain boundary. That is, by obtaining a smoother ion diffusion in the negative electrode, the internal resistance of the electrode is curtailed, and the lithium secondary battery of high capacity is realized also in high rate discharge.
Further researches are also made into the lithium secondary battery using polymer electrolyte, instead of organic electrolyte solution, as the electrolyte, and it is expected as a lithium secondary battery of next generation featuring small size, light weight, and freedom of shape. However, the ion conductivity of polymer electrolyte is about 104 S/cm at most, which is about two digits lower as compared with the organic electrolyte solution. Hence, to obtain a conductivity similar to that of the organic electrolyte solution, gel electrolyte impregnating organic electrolyte solution in polymer is used. The gel electrolyte is manufactured, for example, in the following method as disclosed in Japanese Laid-open Patent No. 5-109310. A mixed solvent composed of an optical crosslinking polymer of polyethylene glycol diacrylate, an optical crosslinking monomer of trimethylol propane ethoxylated triacylate, an electrolyte solution solvent of propylene carbonate or polyethylene oxide, an electrolyte salt of LiCF3SO3, and others is applied on a flat plate, and it is irradiated with electron beams to polymerize and cure the monomer, so that a transparent and soft film of gel electrolyte is obtained. In the gel electrolyte, since ion conduction is mostly conducted through electrolyte solution phase, a high ion conductivity of about 3×105 S/cm can be obtained at room temperature.
In the lithium polymer secondary battery, as the substitute for separator used in the ordinary battery system, the polymer electrolyte is bonded with positive electrode and negative electrode, and the battery is composed. In the case of an ordinary secondary lithium battery using organic electrolyte solution, the positive electrode is composed of active substance, conductive agent and bonding agent, and electrolyte solution is impregnated, so that a favorable electrochemical interface with the active substance is obtained. However, the gel electrolyte which is a solid form lacks fluidity, and hardly permeates into the inside of the electrode. Accordingly, a compound electrode containing polymer electrolyte preliminarily in the electrode is formed, and it is bonded with the polymer electrolyte to fabricate the battery.
However, the conventional gel polymer electrolyte described above is characterized by containing organic electrolyte solution, and although the polymer electrolyte shows a high ion conductivity, it has not reached the level of organic electrolyte solution yet in terms of characteristics. When the battery is composed by introducing this polymer electrolyte into the electrode, owing to the low ion conductivity of the electrolyte itself, the internal resistance of the electrode is increased, and the charging and discharging capacity of the battery is extremely spoiled. Hence, to fabricate the battery of high capacity type, it is required to enhance the ion conductivity of polymer electrolyte in the electrode, and compose a battery of low internal resistance.